There is a decoloring device that carries out a decoloring process on a sheet that has an image formed of a decolorable material, so that the sheet can be reused. However, even if the decoloring process is carried out, the image of the decolorable material may not be completely decolored, and the image may partially remain on the sheet. Such a sheet having an excessive amount of a remaining image may not be suitable for further use, i.e., non-reusable.
When sheets subjected to the decoloring process by the decoloring device are set in a sheet supply cassette of an image forming apparatus, the sheets may include non-reusable sheets, and thus the non-reusable sheets may be used for image forming. Although a use can manually sort out the sheets subjected to the decoloring process into reusable ones and non-reusable ones, it may take significant amount of time and also the sorting by the user may not be accurate.